Healers are kind of OP already, although they do currently eventually run OOM. Also, final numbers are not fixed and healing will be reduced in PvP. PvP power will affect PvP healing too. I'm sure Blizz will put some work into low level, but like all big changes, don't expect them to get it right at first. Also, while low levels are of some interest to them, they will be focusing on 90s and pandas initially.
Same bla bla i always hear, Blizzard can't balance max level let alone low level.